Some layers of materials used in microelectronic and nanoelectronic devices have become extremely thin in recent years, almost to the point of atomic monolayers. For example, in the class of two-dimensional materials, graphene, hexagonal boron nitride and transition metal dichalcogenides are in some cases made of single atomic layers. However, creating useful devices of such small dimensions requires careful patterning and removal of each layer.
Plasma etching is an established method for etching layers in microelectronics. However, although many advances in conventional plasma etching have increased the productivity of known methods by increasing the etch rates, such techniques remain inappropriate for etching extremely thin layers. Furthermore, in conventional plasma etching, the surface of the substrate is often disrupted to a depth of several atomic layers by ion bombardment.
Atomic layer etching techniques (also known as “layer-by-layer dry etching” or “digital etching”) have been developed, which use self-limiting behaviour to remove thin layers of material. Such techniques involve a cycle of formation of a surface layer of partially reacted material in a first step and then removal of just the partially reacted material in a second step.
In one example of a known atomic layer etching technique, doses of chlorine gas are introduced alternately with argon ion bombardment from a shuttered electron cyclotron resonance plasma source. In another known technique based upon plasma etching, it was shown that the window for self-limiting behaviour is very narrow and that changing the DC bias of a radio frequency biased wafer table by just a few volts is sufficient to move between atomic layer etching and conventional etching regimes. Thus, it has also been proposed to precisely control the DC bias on the wafer table either through variation of the magnetic field in magnetically enhanced reaction ion etching or by the use of a complex non-sinusoidal wafer bias scheme.
Subsequently, it has been suggested to use cyclical pulsing schemes, for example with auxiliary electrodes or for example with pulse-biased boundary electrodes to manipulate the plasma potential of an afterglow from a pulsed induction coupled plasma. However, the auxiliary electrode scheme adds complexity and cost to the process chamber. Furthermore, attempts have been made to use the well-defined energy of certain metastable species to drive plasma etching. In some cases, an energetic neutral beam derived from a plasma source has been used for surface excitation in atomic layer etching. In other cases, a cyclical plasma bombardment method has been used for part of a cycle in a self-limiting etch scheme.
However, not one of the above-mentioned etching techniques provide a sufficiently reliable or consistent approach that allows very thin layers of material to be removed.